Methods and system for launching a vehicle

ABSTRACT

Systems and methods for operating a hybrid vehicle driveline that includes an engine and a motor are presented. In one example, the systems and methods include one or more control modes where engine and/or motor speed or torque is adjusted responsive to different control parameters during a vehicle launch from zero speed or a creep speed.

FIELD

The present description relates to methods and a system for controllinga vehicle during launch from a stop or creep. The methods and systemsmay be particularly useful for hybrid vehicles that include a motor anda torque converter having a lockup clutch.

BACKGROUND AND SUMMARY

A driveline of a vehicle may include a torque converter that providestorque multiplication and driveline dampening. The torque multiplicationcan improve vehicle acceleration from zero speed, but the torquemultiplication is a result of operating an engine or motor at a higherspeed to create slip across the torque converter. The engine, motor, andtorque converter may operate less efficiently when there is a largeamount of slip across the torque converter (e.g., a large speeddifferential between a torque converter impeller and torque converterturbine). Consequently, powertrain efficiency may be reduced more thanis desired during vehicle launch. Nevertheless, it may be desirable forthe torque converter to operate with a large amount of slip between thetorque converter impeller and the torque converter turbine when a driveris requesting a large amount of torque so that the vehicle mayaccelerate more rapidly.

The inventors herein have recognized the above-mentioned issue and havedeveloped a driveline operating method, comprising: accelerating atorque converter impeller to a desired speed in response to release of abrake pedal; and maintaining a torque converter impeller speed profileat the desired speed in the presence of an increase in driver demandtorque until a torque converter turbine speed is within a thresholdspeed of the torque converter impeller speed.

By maintaining torque converter impeller speed at a constant or varyingdesired value, it may be possible to provide the technical result ofimproved torque converter and driveline efficiency. The constant torqueconverter impeller speed may allow torque converter turbine speed toapproach torque converter impeller speed sooner so as to reduce torqueconverter slip. Additionally, in some cases where a driver requestsgreater amounts of torque, the device driving the torque converterimpeller may transition between a speed control mode and a torquecontrol mode to improve vehicle acceleration. In some examples, thecontrol of torque flow through the torque converter may be based on avirtual torque converter impeller speed which may improve estimates oftorque transferred through the torque converter.

The present description may provide several advantages. Specifically,the approach may provide more consistent vehicle launches under similaroperating conditions. Further, the approach may improve drivelineefficiency while at the same time providing the capacity to accelerateat a higher rate when requested by driver demand. Further still, theapproach compensates for delays in torque converter clutch lockupoperation so that a driver may experience a more acceptable rate ofvehicle acceleration.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 is a schematic diagram of a hybrid vehicle driveline;

FIG. 3 is an example plot of torque converter steady state efficiency asa function of torque converter slip;

FIGS. 4-11 describe a method for operating a hybrid vehicle driveline;and

FIGS. 12-15 show example vehicle launch sequences during differentconditions.

DETAILED DESCRIPTION

The present description is related to controlling a driveline of ahybrid vehicle that includes a torque converter and torque converterlockup clutch. The hybrid vehicle may include an engine as is shown inFIG. 1. The engine of FIG. 1 may be included in a powertrain ordriveline as is shown in FIG. 2. The driveline may have a torqueconverter having efficiency as shown in FIG. 3. The hybrid vehicle maybe operated according to the method shown in FIGS. 4-11. The hybridvehicle may launch from stopped or creep (e.g., a condition aftervehicle stop where a brake is released and no driver demand has beeninput where the vehicle may move) conditions as shown in the sequencesof FIGS. 12-15.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 is comprised ofcylinder head 35 and block 33, which include combustion chamber 30 andcylinder walls 32. Piston 36 is positioned therein and reciprocates viaa connection to crankshaft 40. Flywheel 97 and ring gear 99 are coupledto crankshaft 40. Starter 96 (e.g., low voltage (operated with less than30 volts) electric machine) includes pinion shaft 98 and pinion gear 95.Pinion shaft 98 may selectively advance pinion gear 95 to engage ringgear 99. Starter 96 may be directly mounted to the front of the engineor the rear of the engine. In some examples, starter 96 may selectivelysupply torque to crankshaft 40 via a belt or chain. In one example,starter 96 is in a base state when not engaged to the engine crankshaft.Combustion chamber 30 is shown communicating with intake manifold 44 andexhaust manifold 48 via respective intake valve 52 and exhaust valve 54.Each intake and exhaust valve may be operated by an intake cam 51 and anexhaust cam 53. The position of intake cam 51 may be determined byintake cam sensor 55. The position of exhaust cam 53 may be determinedby exhaust cam sensor 57. Intake valve 52 may be selectively activatedand deactivated by valve activation device 59. Exhaust valve 54 may beselectively activated and deactivated by valve activation device 58.Valve activation devices 58 and 59 may be electro-mechanical devices.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Fuel injector 66 delivers liquid fuel in proportion to thepulse width from controller 12. Fuel is delivered to fuel injector 66 bya fuel system (not shown) including a fuel tank, fuel pump, and fuelrail (not shown). In one example, a high pressure, dual stage, fuelsystem may be used to generate higher fuel pressures.

In addition, intake manifold 44 is shown communicating with turbochargercompressor 162 and engine air intake 42. In other examples, compressor162 may be a supercharger compressor. Shaft 161 mechanically couplesturbocharger turbine 164 to turbocharger compressor 162. Optionalelectronic throttle 62 adjusts a position of throttle plate 64 tocontrol air flow from compressor 162 to intake manifold 44. Pressure inboost chamber 45 may be referred to a throttle inlet pressure since theinlet of throttle 62 is within boost chamber 45. The throttle outlet isin intake manifold 44. In some examples, throttle 62 and throttle plate64 may be positioned between intake valve 52 and intake manifold 44 suchthat throttle 62 is a port throttle. Compressor recirculation valve 47may be selectively adjusted to a plurality of positions between fullyopen and fully closed. Waste gate 163 may be adjusted via controller 12to allow exhaust gases to selectively bypass turbine 164 to control thespeed of compressor 162. Air filter 43 cleans air entering engine airintake 42.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106 (e.g., non-transitory memory), random access memory 108, keepalive memory 110, and a conventional data bus. Controller 12 is shownreceiving various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including: engine coolanttemperature (ECT) from temperature sensor 112 coupled to cooling sleeve114; a position sensor 134 coupled to an accelerator pedal 130 forsensing force applied by foot 132; a position sensor 154 coupled tobrake pedal 150 for sensing force applied by foot 152, a measurement ofengine manifold pressure (MAP) from pressure sensor 122 coupled tointake manifold 44; an engine position sensor from a Hall effect sensor118 sensing crankshaft 40 position; a measurement of air mass enteringthe engine from sensor 120; and a measurement of throttle position fromsensor 68. Barometric pressure may also be sensed (sensor not shown) forprocessing by controller 12. In a preferred aspect of the presentdescription, engine position sensor 118 produces a predetermined numberof equally spaced pulses every revolution of the crankshaft from whichengine speed (RPM) can be determined.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g., whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC).

During the compression stroke, intake valve 52 and exhaust valve 54 areclosed. Piston 36 moves toward the cylinder head so as to compress theair within combustion chamber 30. The point at which piston 36 is at theend of its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion.

During the expansion stroke, the expanding gases push piston 36 back toBDC. Crankshaft 40 converts piston movement into a rotational torque ofthe rotary shaft. Finally, during the exhaust stroke, the exhaust valve54 opens to release the combusted air-fuel mixture to exhaust manifold48 and the piston returns to TDC. Note that the above is shown merely asan example, and that intake and exhaust valve opening and/or closingtimings may vary, such as to provide positive or negative valve overlap,late intake valve closing, or various other examples.

FIG. 2 is a block diagram of a vehicle 225 including a driveline 200.The driveline of FIG. 2 includes engine 10 shown in FIG. 1. Driveline200 may be powered by engine 10. Engine 10 may be started with an enginestarting system shown in FIG. 1 or via driveline integratedstarter/generator (DISG) 240. DISG 240 (e.g., high voltage (operatedwith greater than 30 volts) electrical machine) may also be referred toas an electric machine, motor, and/or generator. Further, torque ofengine 10 may be adjusted via torque actuator 204, such as a fuelinjector, throttle, etc.

An engine output torque may be transmitted to an input side of drivelinedisconnect clutch 236 through dual mass flywheel 215. Disconnect clutch236 may be electrically or hydraulically actuated. The downstream sideof disconnect clutch 236 is shown mechanically coupled to DISG inputshaft 237.

DISG 240 may be operated to provide torque to driveline 200 or toconvert driveline torque into electrical energy to be stored in electricenergy storage device 275. DISG 240 has a higher output torque capacitythan starter 96 shown in FIG. 1. Further, DISG 240 directly drivesdriveline 200 or is directly driven by driveline 200. There are nobelts, gears, or chains to couple DISG 240 to driveline 200. Rather,DISG 240 rotates at the same rate as driveline 200. Electrical energystorage device 275 (e.g., high voltage battery or power source) may be abattery, capacitor, or inductor. The downstream side of DISG 240 ismechanically coupled to the impeller 285 of torque converter 206 viashaft 241. The upstream side of the DISG 240 is mechanically coupled tothe disconnect clutch 236. DISG may provide torque to wheels 216 whileengine 10 is operating or stopped rotating.

Torque converter 206 includes a turbine 286 to output torque to inputshaft 270. Input shaft 270 mechanically couples torque converter 206 toautomatic transmission 208. Torque converter 206 also includes a torqueconverter bypass lock-up clutch 212 (TCC). Torque is directlytransferred from impeller 285 to turbine 286 when TCC is locked. TCC iselectrically operated by controller 12. Alternatively, TCC may behydraulically locked. In one example, the torque converter may bereferred to as a component of the transmission.

When torque converter lock-up clutch 212 is fully disengaged, torqueconverter 206 transmits engine torque to automatic transmission 208 viafluid transfer between the torque converter turbine 286 and torqueconverter impeller 285, thereby enabling torque multiplication. Incontrast, when torque converter lock-up clutch 212 is fully engaged, theengine output torque is directly transferred via the torque converterclutch to an input shaft 270 of transmission 208. Alternatively, thetorque converter lock-up clutch 212 may be partially engaged, therebyenabling the amount of torque directly relayed to the transmission to beadjusted. The controller 12 may be configured to adjust the amount oftorque transmitted by torque converter 212 by adjusting the torqueconverter lock-up clutch in response to various engine operatingconditions, or based on a driver-based engine operation request.

Automatic transmission 208 includes gear clutches (e.g., gears 1-6) 211and forward clutch 210. The gear clutches 211 (e.g., 1-10) and theforward clutch 210 may be selectively engaged to propel a vehicle.Torque output from the automatic transmission 208 may in turn be relayedto wheels 216 to propel the vehicle via output shaft 260. Specifically,automatic transmission 208 may transfer an input driving torque at theinput shaft 270 responsive to a vehicle traveling condition beforetransmitting an output driving torque to the wheels 216.

Further, a frictional force may be applied to wheels 216 by engagingwheel brakes 218. In one example, wheel brakes 218 may be engaged inresponse to the driver pressing his foot on a brake pedal (not shown).In other examples, controller 12 or a controller linked to controller 12may apply engage wheel brakes. In the same way, a frictional force maybe reduced to wheels 216 by disengaging wheel brakes 218 in response tothe driver releasing his foot from a brake pedal. Further, vehiclebrakes may apply a frictional force to wheels 216 via controller 12 aspart of an automated engine stopping procedure.

Controller 12 may be configured to receive inputs from engine 10, asshown in more detail in FIG. 1, and accordingly control a torque outputof the engine and/or operation of the torque converter, transmission,DISG, clutches, and/or brakes. As one example, an engine torque outputmay be controlled by adjusting a combination of spark timing, fuel pulsewidth, fuel pulse timing, and/or air charge, by controlling throttleopening and/or valve timing, valve lift and boost for turbo- orsuper-charged engines. In the case of a diesel engine, controller 12 maycontrol the engine torque output by controlling a combination of fuelpulse width, fuel pulse timing, and air charge. In all cases, enginecontrol may be performed on a cylinder-by-cylinder basis to control theengine torque output. Controller 12 may also control torque output andelectrical energy production from DISG by adjusting current flowing toand from field and/or armature windings of DISG as is known in the art.Controller 12 receives DISG position via position sensor 271. Controller12 may convert transmission input shaft position into input shaft speedvia differentiating a signal from position sensor 271. Controller 12 mayreceive transmission output shaft torque from torque sensor 272.Alternatively, sensor 272 may be a position sensor, or torque andposition sensors. If sensor 272 is a position sensor, controller 12differentiates a position signal to determine transmission output shaftvelocity. Controller 12 may also differentiate transmission output shaftvelocity to determine transmission output shaft acceleration.

When idle-stop conditions are satisfied, controller 12 may initiateengine shutdown by shutting off fuel and spark to the engine. However,the engine may continue to rotate in some examples. Further, to maintainan amount of torsion in the transmission, the controller 12 may groundrotating elements of transmission 208 to a case 259 of the transmissionand thereby to the frame of the vehicle. When engine restart conditionsare satisfied, and/or a vehicle operator wants to launch the vehicle,controller 12 may reactivate engine 10 by cranking engine 10 andresuming cylinder combustion.

Thus, the system of FIGS. 1 and 2 provides for a driveline, comprising:an engine; a motor; a transmission including a torque converter and atorque converter lockup clutch, the transmission coupled to the motor;and a controller including executable instructions stored innon-transitory memory for adjusting the torque converter lockup clutchin response to a virtual torque converter impeller speed that isdifferent from an actual torque converter impeller speed. The drivelinefurther comprises additional instructions to operate the motor in aspeed control mode in response to driver demand torque being less than athreshold torque during a vehicle launch before a torque converterturbine speed is within a threshold of a torque converter impellerspeed. The driveline further comprises additional instructions tooperate the motor in a torque control mode in response to driver demandtorque being greater than the threshold torque during a vehicle launchbefore the torque converter turbine speed is within the threshold of thetorque converter impeller speed. The driveline includes where thevirtual torque converter impeller speed is based on a driver demand. Thedriveline further comprises instructions to select a torque converterimpeller operating mode in response to a driver demand.

Referring now to FIG. 3, a plot of torque converter steady stateefficiency versus speed ratio of torque converter turbine speed totorque converter impeller speed is shown. The horizontal axis representsa ratio of torque converter turbine speed to torque converter impellerspeed. The vertical axis represents torque converter efficiency. Curve302 describes the relationship between torque converter efficiency andspeed ratio of torque converter turbine speed to torque converterimpeller speed. It may be observed that torque converter efficiencyincreases as the speed ratio of torque converter turbine speed to torqueconverter impeller speed approaches one. The torque converter efficiencydecreases as the speed ratio of torque converter turbine speed to torqueconverter impeller speed approaches zero. A torque converter having aspeed ratio of less than one may be described as slipping. Thus, thedriveline may operate more efficiently if the torque converter speedratio is near a value of one. Also, note that the amount of hydraulictorque transferred from the torque converter impeller to the torqueconverter turbine is limited by the torque converter's hydrauliccharacteristics. The torque converter efficiency is a value of one ifthe torque converter clutch is rigidly locked-up.

Referring now to FIG. 4, a control system block diagram 400 for thesystem of FIGS. 1 and 2 is shown. Torque converter turbine speed isinput to block 402 and accelerator pedal position is input to block 404.The controller outputs control to motive sources 412 (e.g., engine 10and motor 240) and torque converter 414 which includes a torqueconverter lockup clutch.

At block 402, virtual torque converter impeller speed is determinedduring applied torque converter clutch launch (e.g., torque converterclutch starts to transfer friction torque from the torque converterimpeller to the torque converter turbine) by assuming an open torqueconverter (e.g., torque converter clutch is not applied). The virtualtorque converter impeller speed is used to determine a driver demandtorque since driver demand torque is based on torque converter impellerspeed and accelerator pedal position. The virtual torque converterimpeller speed is determined because the actual torque converterimpeller speed in combination with the accelerator pedal position doesnot provide an accurate driver demand torque when the torque converterlockup clutch is being closed since the torque converter impeller speedmay be affected by the torque converter lockup clutch closing. Thetorque converter turbine torque is estimated from the driver demandtorque based on virtual torque converter impeller speed as well as anempirically determined torque converter model. The virtual torqueconverter impeller speed is determined as described in FIG. 6. Thevirtual torque converter impeller speed is provided to blocks 404 and406.

At block 404, accelerator pedal position and virtual torque converterimpeller speed are inputs used to determine a virtual torque converterimpeller torque. In one example, the virtual torque converter impellerspeed and accelerator pedal position are used to index a table orfunction that outputs a desired torque converter impeller torque. Thevirtual torque converter impeller torque is input to block 402.

At block 406, the driver demand torque at the torque converter turbineis estimated. The torque converter turbine hydraulic torque is estimatedby first determining the torque converter impeller torque from thefollowing equation:TQ _(imp-hyd) =f2(ω_(imp) ·R)where TQ_(imp-hyd) is the torque converter hydraulic impeller torque, f2is an empirically determined function describing torque converterhydraulic impeller torque based on a torque converter speed ratio R andtorque converter speed ω_(imp). The torque converter speed ratio isdescribed by the following equation:

$R = \frac{\omega_{turb}}{\omega_{imp}}$where ω_(turb) is the torque converter turbine speed and ω_(imp) is thetorque converter impeller speed. The torque converter turbine torque isdetermined by the following equation:TQ _(turb-hyd) =TQ _(imp-hyd) ·f3(R)where TQ_(turb-hyd) is the torque converter turbine torque and f3 is anempirically determined function that is indexed via the value of R. Theoutput of block 406 is supplied to block 408.

At block 408, a torque splitting strategy determines the amount oftorque which is supplied to the torque converter turbine via thefriction path (e.g., via the TCC) and via the hydraulic path (e.g., viahydraulic fluid in the torque converter). The torque splitting strategydetermines hydraulic path torque and friction path torque based on themethod described in FIGS. 7-10. Block 408 commands the torque converterclutch at block 414 and directs the torque converter hydraulic pathtorque to the impeller torque determining block 410.

At block 410, the desired torque converter impeller torque is determinedfrom the hydraulic path turbine torque, the friction path torque andadditional torque further determined by an engine/motor speed orengine/motor feedback control strategy. As an example, when torqueconverter impeller speed is controlled in a speed control mode viacontrolling engine or motor speed:TQ _(imp-dmd) =TQ _(ff) +TQ _(fb)where TQ_(imp-dmd) is the impeller torque demand, TQ_(ff) is thefeedforward torque command. In one exampleTQ_(ff)=TQ_(turb-hyd)+TQ_(fric), where TQ_(turb-hyd) is the estimatedturbine hydraulic torque and TQ_(fric) is the estimated friction torquefrom converter clutch. TQ_(fb) is the feedback torque command based onspeed feedback, which may be a proportional/integral (PI) controller ora PI controller with inertial compensation or another controllerstrategy.

Block 410 commands the engine and/or motor to supply the desired torqueconverter impeller torque at block 412. Block 412 represents the engineand motor plants which are commanded to provide the torque transmittedvia the hydraulic and friction torque paths. The motor and engine plantsprovide torque to the torque converter impeller at block 414 whichrepresents the torque converter including the torque converter lockupclutch. The torque converter lockup clutch provides torque from thetorque converter friction path to the torque converter turbine by atleast partially closing. The torque converter supplies torque to thetransmission input shaft at block 420 via the torque converter hydraulicpath and the torque converter friction path. The hydraulic and frictiontorques are additive as indicated at summing junction 415.

In this way, torque may be provided to the transmission input shaft viatorque converter hydraulic and friction torque paths. Further, theamount of torque transmitted via the hydraulic and friction torque pathsmay be adjusted based on vehicle operating conditions.

Referring now to FIG. 5, a method for selecting between launching avehicle with an open or an at least partially closed torque converterlockup clutch is shown. The method of FIG. 5 may provide the vehiclelaunch sequences as are shown in FIGS. 12-15. A vehicle launch may bedescribes as a vehicle acceleration from zero speed or creep speed(e.g., vehicle speed after a vehicle is stopped and a brake pedal isreleased without the accelerator pedal being applied) in response to anincrease in accelerator pedal position.

At 502, method 500 determined operating conditions. Operating conditionsmay include but are not limited to vehicle speed, driver demand torque,transmission fluid temperature, engine coolant temperature, and batterystate of charge (SOC). Method 500 proceeds to 504 after operatingconditions are determined.

At 504, method 500 judges if pre-launch conditions are present.Pre-launch conditions may include vehicle speed less than a thresholdspeed and driver demand torque less than a threshold torque. If method500 judges that pre-launch conditions are met, the answer is yes andmethod 500 proceeds to 506. Otherwise, method 500 proceeds to exit.

At 506, method 500 opens the torque converter lockup clutch. The torqueconverter lockup clutch may be initially opened to prepare for abaseline open torque converter lockup clutch. Method 500 proceeds to 508after the torque converter lockup clutch is opened.

At 508, method 500 judges if a driver has applied the accelerator pedalor tipped-in to the accelerator pedal. In one example, method 500 mayjudge that a driver has tipped-in if the accelerator pedal position hasincreased. If method 500 judges that a tip-in has happened or isongoing, the answer is yes and method 500 proceeds to 510. Otherwise,the answer is no and method 500 returns to 508.

At 510, method 500 judges whether or not to apply or at least partiallyclose the torque converter lockup clutch during vehicle accelerationfrom a vehicle speed of zero or a creep speed. The vehicle launch maylast until the driver at least partially releases or reduces theaccelerator pedal. In one example, the torque converter lockup clutchmay be applied when driver demand torque is less than a thresholdtorque, accelerator pedal position is less than a threshold position,torque converter fluid temperature is greater than a first thresholdtemperature and less than a second threshold temperature, and when nodiagnostic degradation of the driveline is determined. If method 500judges to apply the lockup clutch during launch, the answer is yes andmethod 500 proceeds to 512. Otherwise, the answer is no and method 512proceeds to 520.

At 512, method 500 determines an amount of torque to provide to thetorque converter turbine from the torque converter impeller viahydraulic and friction torque paths. The hydraulic torque path transferstorque from the torque converter impeller to the torque converterturbine via hydraulic fluid flowing between the impeller and theturbine. The friction torque path transfers torque from the torqueconverter impeller to the torque converter turbine via the clutchfriction surfaces. Method 500 judges the amount of torque to transfervia the hydraulic and friction paths as is described in FIGS. 6-11.Method 500 proceeds to 514 after determining the amount of torque totransfer to the turbine via hydraulic and friction torque paths.

At 514, method 500 applies the torque converter clutch so that thetorque converter lockup clutch is at least partially applied. The torqueconverter lockup clutch may be closed electrically or hydraulically. Theamount of torque transferred via the torque converter lockup clutch maybe based on a table or function that includes empirically determinedtorque capacity for the torque converter lockup clutch based onhydraulic fluid pressure or electrical current supplied to the torqueconverter clutch. For example, if it is desired for the lockup clutch totransfer 50 Nm of torque, 50 Nm is used to index the torque converterlockup clutch table, and the table outputs a hydraulic line pressure orcurrent supplied that allows the torque converter lockup clutch so thatthe torque converter lockup clutch transfers 50 Nm. Method 500 proceedsto 516 after beginning to apply the torque converter lockup clutch.

At 516, method 500 judges if there is a change in operating conditionssuch that the torque converter lockup clutch is desired open. Forexample, if the driver increases the accelerator pedal position ordriver demand torque to a level greater than a threshold amount, thetorque converter lockup clutch may be desired open to increase torquemultiplication through the torque converter and increased torque flowthrough the torque converter hydraulic torque path. If method 500 judgesthat there is a change and a request to open the torque converter lockupclutch, the answer is yes and method 500 proceeds to 518. Otherwise, theanswer is no and method 500 proceeds to 530.

At 518, method 500 fully opens the torque converter lockup clutch. Thetorque converter lockup clutch may be opened by reducing pressure ofhydraulic fluid supplied to the torque converter lockup clutch or byreducing current supplied to the torque converter lockup clutch. Thetorque converter lockup clutch application force may be ramped out toreduce the possibility of a driveline torque disturbance. Method 500proceeds to 520 after the torque converter lockup clutch is opened.

At 520, method 500 continues the vehicle launch with the torqueconverter clutch fully open. The hydraulic torque path may providetorque multiplication to increase vehicle acceleration. Method 500proceeds to 522 after the torque converter lockup clutch is released.

At 522, method 500 determines torque converter impeller torque based onproviding torque converter turbine torque solely via the hydraulictorque path through the torque converter. In one example, the torqueconverter impeller torque is based on accelerator pedal position andtorque converter impeller speed. The torque converter impeller speed andthe accelerator pedal position are used to index a table or functionthat outputs desired torque converter impeller torque. The desiredtorque converter impeller torque is commanded to the motor and theengine so that the motor and/or engine provide the desired torqueconverter impeller torque. Method 500 proceeds to exit after the motorand engine are commanded to provide the desired torque converterimpeller torque.

At 530, method 500 judges if the conditions for fully locking the torqueconverter lockup clutch are satisfied. In one example, torque converterlockup clutch locking conditions include the torque converter turbinespeed being within a threshold speed of the torque converter impellerspeed. Further, the torque converter impeller speed and the torqueconverter turbine speed may be required to be greater than thresholdspeeds. If conditions for locking the torque converter lockup clutch aresatisfied, the answer is yes and method 500 proceeds to 522. Otherwise,the answer is no and method 500 returns to 512. Thus, method 500provides for changing determination of torque converter turbine torquebased on friction and hydraulic torque paths to being based solely onsingle torque path.

At 532, method 500 transitions to determining desired torque converterimpeller torque based on accelerator pedal position and actual torqueconverter impeller speed. In one example, desired torque converterimpeller torque is ramped from a previous value to a new value based onaccelerator pedal position and actual torque converter impeller speed.If the transition is complete, the answer is yes and method 500 proceedsto 522. Otherwise, the answer is no and method 500 returns to 532 tocontinue transitioning.

Referring now to FIG. 6, a method for determining virtual torqueconverter impeller speed is shown. Virtual torque converter impellerspeed is an estimate of torque converter impeller speed if the torqueconverter lockup clutch was fully open. The virtual torque converterimpeller speed allows for estimating the torque demand at the torqueconverter turbine and further allows for determining the torqueconverter hydraulic path torque and the torque converter friction pathtorque even though the torque converter impeller speed may vary from thevirtual torque converter impeller speed because the torque converterlockup clutch is being applied. This method enables consistent torquedetermination at the turbine side of the torque converter for an opentorque converter launch and applied torque converter launch (e.g.,vehicle launch with the torque converter at least partially applied).

At 602, method 600 determines torque converter impeller torque based ondriver demand torque and torque converter impeller speed. The driverdemand torque may be based on accelerator pedal position. In oneexample, the driver demand torque and the torque converter impellerspeed are used to index a table of empirically determined torqueconverter impeller torque values and the table outputs the torqueconverter impeller torque. Method 600 proceeds to 604 after the torqueconverter impeller torque is determined.

At 604, method 600 judges if it is desired to launch the vehicle bytransferring torque converter impeller torque via two torque paths, afriction torque path (e.g., through the torque converter lockup clutch)and via the torque converter hydraulic path (e.g., though hydraulicfluid between the torque converter impeller and the torque converterturbine). In one example, it may be desirable to launch the vehicle viatransferring torque converter impeller torque through two torque pathswhen driver demand torque is less than a threshold and when acceleratorpedal position is less than a threshold during a vehicle launch. Ifmethod 600 judges to launch the vehicle by transferring torque converterimpeller torque via two torque paths, the answer is yes and method 600proceeds to 606. Otherwise, the answer is no and method 600 returns to602.

At 606, method 600 initializes the virtual torque converter impellerspeed to the present torque converter impeller speed as determined via aspeed sensor sensing motor and/or engine speed. Thus, the virtualimpeller speed may be expressed as VirtImpSpd(k)=present actual torqueconverter impeller speed, where k is the number of times the virtualtorque converter impeller speed has been determined. Method 600 proceedsto 608 after the initial virtual torque converter impeller speed isdetermined.

At 608, method 600 determines a virtual driver demand based on thevirtual impeller speed and the accelerator pedal position. The virtualdriver demand may be expressed as VirtDmd(k)=f1(VirtImpSpd(k),ActPed(k)), where f1 is a table of empirically determined driver demandvalues and ActPed is the actual accelerator pedal position. Method 600proceeds to 610 after the virtual driver demand is determined.

At 610, method 600 determines the virtual torque converter impellerhydraulic torque. The virtual torque converter impeller hydraulic torquemay be expressed as TQimp_hyd_virt(k)=f2(VirtImpSpd(k), ActTurbSpd(k)),where TQimp_hyd_virt is the virtual torque converter impeller hydraulictorque, f2 is a table or function of empirically determined virtualtorque converter impeller hydraulic torque values, ActTurbSpd is thepresent actual torque converter turbine speed which may be sensed via aspeed or position sensor. Method 600 proceeds to 612 after the virtualtorque converter impeller hydraulic torque is determined.

At 612, method 600 determines virtual torque converter impelleracceleration. The virtual torque converter impeller acceleration may beexpressed as VirtImpAcc(k)=(VirtDmd(k)−TQimp_hyd_virt(k))/Jimp, whereJimp is torque converter impeller inertia. Method 600 proceeds to 614after the virtual torque converter impeller acceleration is determined.

At 614, method 600 determines virtual impeller speed one event into thefuture. The virtual impeller speed one event in the future is expressedas VirtImpSpd(k+1)=VirtImpAcc(k)*Δt+VirtImpSpd(k). Method 600 proceedsto 616 after the virtual impeller speed one event in the future isdetermined.

At 616, method 600 judges if it is desired to determine torque converterturbine torque based on a single torque path (e.g., the hydraulic torquepath). Method 600 may judge that it is desirable to determine torqueconverter turbine torque based on a single torque path in response todriver demand torque being greater than a predetermined threshold. Ifmethod 600 judges it is desirable to determine torque converter turbinetorque based on a single torque path, the answer is yes and method 600exits. Otherwise, the answer is no and method 600 returns to 608.

Thus, method 600 determines virtual torque converter impeller speed forthe present event and one event into the future. The torque converterimpeller torque is determined from the virtual torque converter impellerspeed. In particular, torque converter impeller torque is determined viathe equation TQ_(imp-hyd)=f2(ω_(imp)·R) described at 406 of FIG. 4.

Referring now to FIG. 7, a method for controlling a motor and/or anengine in a speed or torque control mode during vehicle launch is shown.The method of FIG. 7 may be included in the method of FIG. 5.

At 702, method 700 determines whether to control the torque converterimpeller in a speed control mode or a torque control mode. Method 700judges whether to operate the torque converter impeller in a speedcontrol or torque control mode based on the method of FIG. 11. Thetorque converter impeller is operated in a speed control or torquecontrol mode by operating the motor and/or engine in speed or torquecontrol mode. In torque control mode, engine and/or motor torque iscontrolled to a desired torque and engine and/or motor speed is allowedto vary to provide the desired torque. In speed control mode, engineand/or motor speed is controlled to a desired speed and engine and/ormotor torque is allowed to vary to provide the desired speed. Method 700proceeds to 704 after it is determined whether to operate in speed ortorque control mode.

At 704, method 700 provides feedback control of torque converterimpeller speed or torque. If it is desired to operate the torqueconverter impeller in speed control mode, the engine and/or motor torqueare adjusted to operate the torque converter impeller at the desiredspeed. The torque converter impeller speed is feedback to the controllerand torque converter impeller speed adjustments are determined if torqueconverter impeller speed is greater or less than a desired speed.Likewise, if it is desired to operate the torque converter impeller intorque control mode, the engine and/or motor torque are adjusted tooperate the torque converter impeller at the desired torque. The torqueconverter impeller torque is feedback to the controller and torqueconverter impeller torque adjustments are determined if torque converterimpeller torque is greater or less than a desired torque. The torqueconverter impeller torque may be sensed via a torque sensor or inferred.Alternatively, the torque converter impeller torque may be controlledwithout torque feedback or torque inference. Method 700 proceeds to 706after the torque converter impeller speed or torque is feedbackadjustments are determined.

At 706, method 700 commands the torque converter lockup clutch toprovide a desired amount of torque transfer capacity from the torqueconverter impeller to the torque converter turbine via the torqueconverter lockup clutch. The torque converter lockup torque capacity isadjusted by adjusting a force applied to the torque converter lockupclutch. Further, the motor and/or engine torque are commanded based onthe desired torque converter impeller speed or torque and feedbackadjustments. The desired torque converter impeller speed or torque mayvary with vehicle operating conditions. The engine torque or speed isadjusted via adjusting an engine torque actuator such as a throttle,spark timing, cam timing, or fuel amount. The motor torque or speed isadjusted via adjusting an amount of current supplied to the motor.Method 700 proceeds to exit after the engine, motor, and torqueconverter lockup clutch are adjusted.

Referring now to FIG. 8, a method for operating the engine and/or motorin speed control mode while providing a desired torque converter turbinetorque is shown. It may be desirable to operate the engine and/or motorin a speed control mode to increase torque converter efficiency ascompared to operating the torque converter in a torque control modewhere torque converter impeller speed may be much different than torqueconverter turbine speed.

At 802, method 800 operates the engine and/or motor in speed controlmode to control torque converter impeller speed to a desired speed. Thedesired speed may vary depending on vehicle operating conditions. Forexample, the desired impeller speed may be adjusted to a minimum speedat which a transmission pump outputs a desired output pressure tooperate transmission clutches. Alternatively, the desired impeller speedmay be adjusted to a base engine idle speed that is greater than theminimum speed at which the transmission pump outputs the desired outputpressure. In still other examples, when the transmission, engine, orother driveline component is cold, the desired impeller speed may beadjusted to a speed greater than the base engine idle speed. The engineand/or motor torque may vary when the engine and/or motor speed iscontrolled to the desired speed. The torque converter impeller speed mayalso be increased in response to torque converter lockup clutch delay soas to increase torque converter slippage via increasing the speeddifference between torque converter impeller speed and torque converterturbine speed as is shown in FIG. 14. Alternatively, a desired slipamount (e.g., a difference between torque converter impeller speed andtorque converter turbine speed) may be determined based on operatingconditions. The torque converter impeller speed may be commanded toprovide the desired amount of slip. Method 800 proceeds to 804 afterinitiating engine and/or motor in speed control mode.

At 804, method 800 estimates torque converter turbine torque demand andactual torque converter impeller hydraulic torque. The actual torqueconverter impeller hydraulic torque is determined from the followingequation:TQ _(imp-hyd) =f2(ω_(imp) ·R)where TQ_(imp-hyd) is the actual torque converter impeller torque, f2 isan empirically determined function describing torque converter impellertorque based on a torque converter speed ratio R and torque converterimpeller speed ω_(imp). The torque converter speed ratio is described bythe following equation:

$R = \frac{\omega_{turb}}{\omega_{imp}}$where ω_(turb) is the torque converter turbine speed and ω_(imp) is thetorque converter impeller speed. The demand torque converter turbinetorque TQ_(turb) _(_) _(dmd) is determined by the following equation:TQ _(turb) _(_) _(dmd) =f2(ω_(imp-virt) ·R _(virt))*f3(R _(virt))where TQ_(turb) _(_) _(dmd) is the estimated driver demand at torqueconverter turbine, f2(ω_(imp-virt)·R_(virt)) is the estimated hydraulictorque at torque converter impeller with virtual impeller speedω_(imp-virt) and slip ratio R_(virt)·f3(R_(virt)) is the torquemultiplication ratio for torque converter. R_(virt) is the virtual slipratio defined as:

$R_{virt} = \frac{\omega_{turb}}{\omega_{imp\_ virt}}$f2 and f3 are empirically determined functions that are indexed via thevalue of R. Method 800 proceeds to 806 after the torque converterturbine torque demand and actual impeller hydraulic torque areestimated.

At 806, method 800 adjusts the torque converter lockup clutchapplication force to provide a torque converter lockup clutch capacity(e.g., an amount of torque the torque converter may transfer between theimpeller and the turbine). In particular, the torque converter lockupclutch may be adjusted based on the following equations:

$\begin{matrix}{{TQ}_{fric\_ dmd} = {{TQ}_{turb\_ dmd} - {{TQ}_{imp\_ hyd}*f\; 3(R)}}} \\{= {{f\; 2\left( {\omega_{{imp} - {virt}} \cdot R_{virt}} \right)*f\; 3\left( R_{virt} \right)} - {f\; 2\left( {\omega_{imp} \cdot R} \right)*f\; 3(R)}}}\end{matrix}$where the variables are described at 804. Method 800 proceeds to exitafter the torque converter lockup clutch torque capacity is adjusted.

Referring now to FIG. 9, a method for operating the motor and/or enginein torque and speed control modes during vehicle launch is shown. Byoperating the engine and/or motor in torque and speed control modesduring vehicle launch, it may be possible to provide improve torqueresponse while maintaining a higher torque converter efficiency.

At 902, method 900 operates the motor and/or engine in a torque controlmode. In torque control mode, engine and/or motor torque is controlledto a desired torque and engine and/or motor speed varies depending onthe load applied to the motor and/or engine. The engine torque isadjusted to a desired torque via adjusting an engine torque actuatorsuch as a throttle, cam, spark timing, or fuel injection amount. Themotor torque is adjusted via adjusting an amount of current supplied tothe motor. The motor and engine torque are adjusted to provide at torquebased on accelerator pedal position and torque converter impeller speed.In particular, accelerator pedal position and torque converter impellerspeed index a table or function of empirically determined torqueconverter impeller torque amounts. The table or function outputs thetorque amount and the engine and/or motor torque is adjusted to providethe torque output from the table or function. Method 900 proceeds to 904after engine and/or motor torque is adjusted.

At 904, method 900 applies the torque converter lockup clutch to begintransferring torque converter impeller torque to the torque converterturbine via the friction torque path. The amount of torque transferredvia the torque converter lockup clutch may be estimated by hydraulicpressure or electrical current supplied to the torque converter lockupclutch. In one example, a table of empirically determined torqueconverter lockup clutch torque capacity values is indexed based onhydraulic pressure supplied to the torque converter lockup clutch. Thetable outputs an estimate of the torque converter lockup clutch torquecapacity (e.g., an amount of torque the torque converter lockup clutchmay transfer). Method 900 proceeds to 906 after the torque converterclutch lockup capacity is determined.

At 906, method 900 judges if the torque converter lockup clutch torquedelay has been overcome. In one example, method 900 judges that thetorque converter lockup clutch torque delay has been overcome when thetorque converter lockup torque capacity exceeds a threshold value. Ifmethod 900 judges that the torque converter lockup clutch delay has beenovercome, the answer is yes and method 900 proceeds to 908. Otherwise,the answer is no and method 900 returns to 902.

At 908, method 900 operates the engine and/or motor in speed controlmode to control torque converter impeller speed to a desired speed. Thedesired speed may vary depending on vehicle operating conditions. Forexample, the desired impeller speed may be adjusted to a minimum speedat which a transmission pump outputs a desired output pressure tooperate transmission clutches. Alternatively, the desired impeller speedmay be adjusted to a base engine idle speed that is greater than theminimum speed at which the transmission pump outputs the desired outputpressure. In still other examples, when the transmission, engine, orother driveline component is cold, the desired impeller speed may beadjusted to a speed greater than the base engine idle speed. The engineand/or motor torque may vary when the engine and/or motor speed iscontrolled to the desired speed. Alternatively, a desired slip amount(e.g., a difference between torque converter impeller speed and torqueconverter turbine speed may be determined based on operating conditions.The torque converter turbine speed may be commanded to provide thedesired amount of slip. Method 900 proceeds to 910 after initiatingengine and/or motor in speed control mode.

At 910, method 900 estimates torque converter turbine demand torque andactual torque converter impeller hydraulic torque. The torque converterimpeller hydraulic torque is estimated from the following equation:TQ _(imp-hyd) =f2(ω_(imp) ·R)where TQ_(imp-hyd) is the torque converter impeller torque, f2 is anempirically determined function describing torque converter impellertorque based on a torque converter speed ratio R and torque converterimpeller speed ω_(imp). The torque converter speed ratio is described bythe following equation:

$R = \frac{\omega_{turb}}{\omega_{imp}}$where ω_(turb) is the torque converter turbine speed and ω_(imp) is thetorque converter impeller speed. The demand torque converter turbinetorque TQ_(turb) _(_) _(dmd) is determined by the following equation:TQ _(turb) _(_) _(dmd) =f2(ω_(imp-virt) ·R _(virt))*f3(R _(virt))where TQ_(turb) _(_) _(dmd) is the estimated driver demand at torqueconverter turbine, f2(ω_(imp-virt)·R_(virt)) is the estimated hydraulictorque at torque converter impeller with virtual impeller speedω_(imp-virt) and slip ratio R_(virt)·f3(R_(virt)) is the torquemultiplication ratio for torque converter. R_(virt) is the virtual slipratio defined as:

$R_{virt} = \frac{\omega_{turb}}{\omega_{imp\_ virt}}$f2 and f3 are empirically determined functions that are indexed via thevalue of R. Method 900 proceeds to 912 after the torque converterturbine demand torque and actual impeller hydraulic torque areestimated.

At 912, method 900 adjusts the torque converter lockup clutchapplication force to provide a torque converter lockup clutch capacity(e.g., an amount of torque the torque converter may transfer between theimpeller and the turbine). In particular, the torque converter lockupclutch may be adjusted based on the following equations:

$\begin{matrix}{{TQ}_{fric\_ dmd} = {{TQ}_{turb\_ dmd} - {{TQ}_{imp\_ hyd}*f\; 3(R)}}} \\{= {{f\; 2\left( {\omega_{{imp} - {virt}} \cdot R_{virt}} \right)*f\; 3\left( R_{virt} \right)} - {f\; 2\left( {\omega_{imp} \cdot R} \right)*f\; 3(R)}}}\end{matrix}$where the variables are as described at 804. The torque converter lockupclutch application force is adjusted via adjusting current or hydraulicpressure supplied to the torque converter lockup clutch, and theapplication force adjusts the torque converter lockup clutch torquecapacity to the torque converter friction path torque amount TQ_(frict).Method 900 proceeds to exit after the torque converter lockup clutchtorque capacity is adjusted.

Referring now to FIG. 10, a method is shown for operating a motor and/orengine in torque control mode and adjusting application force of thetorque converter lockup clutch to provide a desired torque converterimpeller speed or a desired amount of slip (e.g., speed difference)between a torque converter impeller and a torque converter turbine.

At 1002, method 1000 operates the motor and/or engine in a torquecontrol mode. In torque control mode, engine and/or motor torque iscontrolled to a desired torque and engine and/or motor speed variesdepending on the load applied to the motor and/or engine. The enginetorque is adjusted to a desired torque via adjusting an engine torqueactuator such as a throttle, cam, spark timing, or fuel injectionamount. The motor torque is adjusted via adjusting an amount of currentsupplied to the motor. The motor and engine torque are adjusted toprovide at torque based on accelerator pedal position and torqueconverter impeller speed. Specifically, torque converter impeller torqueis estimated based on the virtual torque converter impeller speed aspreviously described. Method 1000 proceeds to 1004 after engine and/ormotor torque is adjusted.

At 1004, method 1000 adjusts the motor and/or engine torque demand basedon the following equation:

${TQ}_{imp\_ dmd} = {\frac{{TQ}_{turb\_ dmd} - {TQ}_{fric}}{f\; 3(R)} + {TQ}_{fric}}$where TQ_(mot/eng) is the commanded motor and/or engine torque,TQ_(turb) _(_) _(dmd) is the torque converter turbine torque demand,TQ_(fric) is the estimated torque converter friction path torque (e.g.,torque converter lockup clutch torque transferring capacity), and f3(R)is the torque converter torque ratio as a function of slip ratio R. R isdefined by the equation:

$R = \frac{\omega_{turb}}{\omega_{imp}}$where ω_(turb) is the turbine speed and ω_(imp) is the impeller speed.Method 1000 proceeds to 1006 after commanding motor and/or enginetorque.

At 1006, method 1000 determines the desired torque converter impellerspeed or desired torque converter slip amount. The desired torqueconverter impeller speed may vary depending on vehicle operatingconditions. For example, the desired impeller speed may be adjusted to aminimum speed at which a transmission pump outputs a desired outputpressure to operate transmission clutches. Alternatively, the desiredimpeller speed may be adjusted to a base engine idle speed that isgreater than the minimum speed at which the transmission pump outputsthe desired output pressure. In still other examples, when thetransmission, engine, or other driveline component is cold, the desiredimpeller speed may be adjusted to a speed greater than the base engineidle speed. Alternatively, a desired slip amount (e.g., a differencebetween torque converter impeller speed and torque converter turbinespeed) may be determined based on operating conditions. The torqueconverter impeller speed may be commanded to provide the desired amountof slip. Method 1000 proceeds to 1008 after initiating engine and/ormotor in speed control mode.

At 1008, method 1000 adjusts the torque converter lockup clutchapplication force to provide the desired torque converter impeller speedor torque converter slip. For example, if the desired torque converterspeed is 400 RPM and the present torque converter impeller speed is 500RPM, the torque converter lockup clutch application force is increasedto slow the torque converter impeller and transfer torque from thetorque converter impeller to the torque converter turbine. Likewise, ifthe desired torque converter speed is 500 RPM and the present torqueconverter impeller speed is 400 RPM, the torque converter lockup clutchapplication force is decreased to accelerate the torque converterimpeller and reduce torque transferred from the torque converterimpeller to the torque converter turbine. The torque converter lockupclutch application force may also be adjusted in a similar way toincrease or decrease torque converter slip so that a desired slip amountis provided. Additionally, the torque demand for the torque converterlockup clutch is limited to be less than the estimated turbine torquedemand. Method 1000 proceeds to exit after adjusting the torqueconverter lockup clutch application force and torque capacity.

Referring now to FIG. 11, a method for selecting a vehicle launchcontrol mode is shown. The control mode may be a speed control mode, atorque control mode, or a combination of speed and torque control modes.

At 1102, method 1100 judges if a brake pedal is released after a vehiclestop. The engine and/or motor may be stopped when the vehicle is stoppedto conserve energy. The brake pedal may be determined to be released viaa signal from a brake pedal sensor. If method 1100 judges that the brakepedal is released after a vehicle stop, the answer is yes and method1100 proceeds to 1104. Otherwise, the answer is no and method 1100proceeds to exit.

At 1104, method 1100 operates the motor and/or engine in a speed controlmode (e.g., torque converter impeller speed control) while waiting for atip-in (e.g., increase in accelerator pedal position) to initiatevehicle launch. The motor and/or engine may be operated at differentselected speeds based on operating conditions. For example, the motormay be operated at a minimum speed for a transmission pump to supplytransmission fluid at a desired pressure. Such a speed may be a lowestcommanded torque converter impeller speed. Additionally, the motorand/or engine may operate at a base engine idle speed (e.g., 800 RPM) ora speed greater than a base idle speed when the engine temperature isless than a threshold. Method 1100 proceeds to 1106 after the motorand/or engine are commanded to a desired speed in a speed control mode.

At 1106, method 1100 judges if a tip-in has occurred. A tip-in may bedetermined based on a change in accelerator pedal position. If method1100 judges that a tip-in has occurred, the answer is yes and method1100 proceeds to 1108. Otherwise, the answer is no and method 1100returns to 1104.

At 1108, method 1100 judges if it is desired to apply the torqueconverter lockup clutch (TCC) during vehicle launch. An open torqueconverter lockup clutch may be desired in response to a tip-in oraccelerator pedal position greater than a first threshold amount. Alarge torque request or accelerator pedal position may be indicated by atip-in greater than the first threshold amount. Greater torquemultiplication may be desired during a greater tip-in to accelerate thevehicle more quickly. If method 1100 judges that a tip-in greater than afirst threshold amount or accelerator pedal position greater than afirst threshold amount, it may be judged that it is not desirable toapply the TCC during launch. Therefore, the answer is no and method 1100proceeds to 1110. Otherwise, the answer is yes and method 1100 proceedsto 1112.

At 1110, the torque converter lockup clutch is held open to provide amaximum amount of torque multiplication to accelerate the vehicle. Theopen torque converter lockup clutch may reduce torque converterefficiency, but the vehicle may accelerate more quickly. Method 1100proceeds to exit after the torque converter lockup clutch is opened.Note that the torque converter lockup clutch may be closed after torqueconverter turbine speed is within a threshold speed of torque converterimpeller speed after or during vehicle launch.

At 1112, method 1100 judges if it is desired to transition the torqueconverter impeller, and motor and/or engine to a torque control modewhile applying the torque converter lockup clutch. The torque converterimpeller is in speed control when the engine and/or motor are in speedcontrol. The torque converter impeller is in torque control when theengine and/or motor are in torque control. By operating the impeller andmotor and/or engine in torque control mode, the torque converterimpeller may be adjusted to a torque based on the driver demand torqueso that additional torque may be transferred to the torque converterimpeller via the hydraulic or friction torque path. In one example,method 110 judges that it may be desired to operate the impeller andmotor and/or engine in a torque control mode when the driver demandtorque or accelerator pedal position is less greater than a secondthreshold, the second threshold less than the first threshold. If method1100 judges that a tip-in greater than a second threshold amount oraccelerator pedal position greater than a second threshold amount, itmay be judged that it is desired to operate the torque converterimpeller and motor and/or engine in a torque control mode. Therefore,the answer is yes and method 1100 proceeds to 1114. Otherwise, theanswer is no and method 1100 proceeds to 1116.

At 1114, method 1100 operates the torque converter impeller in a torquecontrol mode while applying the TCC as described in FIG. 9 or 10. In oneexample, the method of FIG. 9 is selected in response to a first groupof operating conditions, and the method of FIG. 10 is selected inresponse to a second group of operating conditions. Method 1100 proceedsto exit after the torque converter impeller and motor and/or engine areoperated in a torque control mode during vehicle launch.

At 1116, method 1100 judges if a desired torque converter impeller speedis less than (L.T.) a threshold speed. The desired torque converterspeed may be a minimum transmission pump speed to provide transmissionfluid at a minimum desired pressure, a base engine idle speed, or aspeed greater than a base engine idle speed. Further, if the tip-inevent occurs before the torque converter impeller reaches a desiredtorque converter impeller speed, the desired torque converter speed maybe adjusted to the present torque converter impeller speed. However, ifthe present torque converter impeller speed is less than a minimum speedfor the transmission pump to output a desired pressure, the desiredtorque converter speed is adjusted to the minimum transmission pumpspeed to provide the desired pressure. If method 1100 judges thatdesired torque converter impeller speed is less than a threshold, theanswer is yes and method 1100 proceeds to 1120. Otherwise, the answer isno and method 1100 returns to 1118.

At 1120, the torque converter impeller is operated in a speed controlmode where the torque converter impeller is rotate by the motor and themotor is operated at a minimum speed where the transmission fluid pumpsupplies transmission fluid at a minimum desired pressure as describedin FIG. 8. Method 1100 proceeds to exit after the motor and torqueconverter impeller are operated during speed control during the vehiclelaunch.

At 1118, the torque converter impeller is operated in a speed controlmode where the torque converter impeller is rotate by the motor orengine at a speed greater than the minimum speed where the transmissionfluid pump supplies transmission fluid at a minimum desired pressure asdescribed in FIG. 8. Method 1100 proceeds to exit after the motor andtorque converter impeller are operated during speed control during thevehicle launch.

In this way, the method of FIG. 11 selects between operating the torqueconverter in a speed control mode or a torque control mode. Operatingthe torque converter impeller in speed control mode may increase torqueconverter and driveline efficiency. On the other hand, operating thetorque converter impeller in a torque control mode while at the sametime the torque converter lockup clutch is applied may improve vehicleacceleration and driveline efficiency.

Thus, the method of FIGS. 5-11 provide for a driveline operating method,comprising: accelerating a torque converter impeller to a desired speedin response to release of a brake pedal; and maintaining a torqueconverter impeller speed at the desired speed in the presence of anincrease in driver demand torque until a torque converter turbine speedis within a threshold speed of the torque converter impeller speed. Themethod includes where a motor operating in a speed control mode rotatesthe torque converter impeller. The method includes where the desiredspeed is a minimum transmission pump speed to provide a desiredtransmission fluid pressure. The method includes where an engineoperating in a speed control mode rotates the torque converter impeller.

In some examples, the method further comprises at least partiallyclosing a torque converter lockup clutch while maintaining the torqueconverter impeller speed at the desired speed. The method furthercomprises increasing the torque converter impeller speed in response tothe torque converter turbine speed being within the threshold speed ofthe torque converter impeller speed. The method further comprises fullyclosing the torque converter lockup clutch in response to the torqueconverter turbine speed being within a threshold speed of the torqueconverter impeller speed.

The method of FIGS. 5-11 also provide for a driveline operating method,comprising: accelerating a torque converter impeller to a desired speedin response to a brake pedal release; maintaining a torque converterimpeller speed at the desired speed until an increase in driver demandtorque; entering a torque converter impeller torque control mode duringa torque converter lockup clutch delay period; and entering a torqueconverter impeller speed control mode in response to the torqueconverter lockup clutch delay period elapsing or passing. The methodfurther comprises entering the torque converter impeller torque controlmode in response to the torque converter impeller speed being within athreshold speed of a torque converter turbine speed. The method includeswhere the desired speed is a base engine idle speed. The method includeswhere in the desired speed is a minimum transmission pump speed toprovide a desired transmission fluid pressure.

In some examples, the method includes where the torque converter lockupclutch delay period is from a time when a torque converter lockup clutchis commanded to a time when the torque converter lockup clutch starts toapply force to transfer torque across the torque converter. The methodfurther comprises fully locking a torque converter lockup clutch inresponse to the torque converter impeller speed being within a thresholdspeed of a torque converter turbine speed. The method further comprisesat least partially closing a torque converter lockup clutch in responseto an increase in driver demand. The method further comprises adjustingthe torque converter lockup clutch in response to a virtual torqueconverter impeller speed.

Referring now to FIG. 12, plots of torque converter impeller speed andtorque converter turbine speed during vehicle launch is shown. For thefirst plot at the top of FIG. 12, the horizontal axis represents timeand time increases from the left to right side of the figure. Thevertical axis represents speed and speed increases in the direction ofthe vertical axis arrow. Horizontal line 1202 represents a desiredtorque converter impeller speed. Line 1204 represents torque converterimpeller speed. Line 1206 represents torque converter turbine speed. Forthe second plot from the top of FIG. 12, the horizontal axis representstime and the vertical axis represent torque converter clutch torquetransfer capacity. Line 1208 represents torque converter lockup clutchtransfer capacity. The time scales of the first plot and the second plotrepresent the same time, and the vertical lines T1-T3 represent time ofinterest in the launch sequence.

Before time T1 the torque converter impeller speed and torque converterturbine speed are zero indicating that the engine and motor are stopped.Further, the vehicle is stopped and the torque converter lockup clutchis open as is indicated by the lockup clutch torque transfer capacitybeing low. At time T1, the driver releases a vehicle brake pedal (notshown) in preparation for vehicle launch. Shortly thereafter, the torqueconverter impeller speed is accelerated to the level of line 1202 whilethe torque converter lockup clutch remains open. In this example, thetorque converter turbine does not rotate when torque converter impellerreaches the amount or level of line 1202, but in other examples thetorque converter turbine may rotate before the driver tips-in.

At time T2, the driver applies the accelerator pedal (not shown) tolaunch the vehicle. The torque converter impeller speed is maintainedconstant at the level of line 1202 and the torque converter turbinespeed begins to increase as the torque converter lockup clutch begins toclose in response to the tip-in. The torque converter lockup clutchbegins to transfer torque from the torque converter impeller to thetorque converter turbine and force is applied to close the torqueconverter lockup clutch. The torque converter impeller speed remainsunchanged since the motor and/or engine are in speed control mode. Themotor and/or engine torque output may increase to maintain the torqueconverter impeller at a constant speed since the torque converter lockupclutch is transferring torque from the torque converter impeller to thetorque converter turbine.

At time T3, the torque converter turbine speed reaches the torqueconverter impeller speed. The torque converter impeller and motor and/orengine exit speed control mode and enter torque control mode wheredriver demand torque is the basis for adjusting torque converterimpeller torque. Further, the torque converter lockup clutch torquetransfer capacity is increased to fully lock the torque converter lockupclutch, thereby increasing torque converter torque transfer capacity.

In this way, torque converter impeller speed may be maintained duringlaunch to improve torque converter efficiency. Further, torque converterimpeller speed control mode may be exited in response to torqueconverter turbine speed being within a threshold speed of torqueconverter impeller speed.

Referring now to FIG. 13, plots of torque converter impeller speed andtorque converter turbine speed during vehicle launch is shown. For thefirst plot at the top of FIG. 13, the horizontal axis represents timeand time increases from the left to right side of the figure. Thevertical axis represents speed and speed increases in the direction ofthe vertical axis arrow. Dashed horizontal line 1302 represents aninitial desired torque converter impeller speed, in this example a speedelevated from a base engine idle speed. Solid line 1304 representsactual torque converter impeller speed. Line 1308 represents torqueconverter turbine speed. Line 1306 represents a minimum speed torqueconverter impeller speed where a transmission fluid pump suppliestransmission fluid at a desired pressure. Line 1310 represents a torqueconverter impeller speed trajectory if a tip-in is not provided beforetorque converter impeller speed reaches the initial desired torqueconverter impeller speed. The torque converter impeller speedrepresented by line 1310 is equivalent to torque converter impellerspeed represented by line 1304 until a tip-in occurs at time T6. For thesecond plot from the top of FIG. 13, the horizontal axis represents timeand the vertical axis represent torque converter clutch torque transfercapacity. Line 1312 represents torque converter lockup clutchtransferred torque. The time scales of the first plot and the secondplot represent the same time, and the vertical lines T5-T7 representtime of interest in the launch sequence.

Before time T5 the torque converter impeller speed and torque converterturbine speed are zero indicating that the engine and motor are stopped.Further, the vehicle is stopped and the torque converter lockup clutchis open as is indicated by the lockup clutch torque transfer capacitybeing low. At time T5, the driver releases a vehicle brake pedal (notshown) in preparation for vehicle launch. Shortly thereafter, the torqueconverter impeller speed is accelerated above the level of line 1306 sothat a minimum transmission output pressure is provided by the torqueconverter impeller spinning the transmission fluid pump. The torqueconverter lockup clutch remains open. In this example, the torqueconverter turbine does not rotate when torque converter impeller reachesthe amount or level of line 1306, but in other examples the torqueconverter turbine may rotate before the driver tips-in.

At time T6, the driver applies the accelerator pedal (not shown) tolaunch the vehicle. The desired torque converter impeller speed becomesthe torque converter impeller speed at the time the driver tips-ininstead of the initial desired torque converter impeller speed 1302 sothat torque transfer through the torque converter may be smooth.However, if the actual torque converter impeller speed were less thanthe minimum pump speed for the transmission pump to output a desiredpressure, the desired torque converter speed would be adjusted to theminimum pump speed. The torque converter turbine speed begins toincrease as the torque converter lockup clutch begins to close inresponse to the tip-in. The torque converter lockup clutch begins totransfer torque from the torque converter impeller to the torqueconverter turbine and force is applied to close the torque converterlockup clutch. The torque converter impeller speed remains unchangedafter the tip-in since the motor and/or engine are in speed controlmode. The motor and/or engine torque output may increase to maintain thetorque converter impeller at a constant speed since the torque converterlockup clutch is transferring torque from the torque converter impellerto the torque converter turbine.

At time T7, the torque converter turbine speed reaches the torqueconverter impeller speed. The torque converter impeller and motor and/orengine exit speed control mode and enter torque control mode wheredriver demand torque is the basis for adjusting torque converterimpeller torque. Further, the torque converter lockup clutch torquetransfer capacity is increased to fully lock the torque converter lockupclutch, thereby increasing torque converter efficiency.

In this way, torque converter impeller speed may be maintained at one ofseveral different levels in response to vehicle operating conditionsduring launch to improve torque converter efficiency. Further, torqueconverter impeller speed control mode may be exited and a torqueconverter impeller torque control mode entered in response to torqueconverter turbine speed being within a threshold speed of torqueconverter impeller speed.

Referring now to FIG. 14, plots of torque converter impeller speed andtorque converter turbine speed during vehicle launch is shown. For thefirst plot at the top of FIG. 14, the horizontal axis represents timeand time increases from the left to right side of the figure. Thevertical axis represents speed and speed increases in the direction ofthe vertical axis arrow. Line 1402 represents a baseline engine idlespeed and line 1404 represents a minimum transmission pump speed toprovide a desired transmission fluid pressure. Dashed line 1406represents torque converter impeller speed. Solid line 1408 representstorque converter turbine speed. For the second plot from the top of FIG.14, the horizontal axis represents time and the vertical axis representtorque converter clutch torque transfer capacity or torque converterslip speed. Dashed line 1412 represents torque converter lockup clutchtorque transfer capacity, and solid line 1410 represents torqueconverter slip speed. The time scales of the first plot and the secondplot represent the same time, and the vertical lines T10-T13 representtime of interest in the launch sequence.

Before time T10 the torque converter impeller speed and torque converterturbine speed are zero indicating that the engine and motor are stopped.Further, the vehicle is stopped and the torque converter lockup clutchis open as is indicated by the lockup clutch application torque transfercapacity being low. At time T10, the driver releases a vehicle brakepedal (not shown) in preparation for vehicle launch. Shortly thereafter,the torque converter impeller speed is accelerated while the torqueconverter impeller and motor and/or engine are in a speed control modein response to vehicle operating conditions while the torque converterlockup clutch remains open. In this example, the torque converterturbine does not rotate when torque converter impeller accelerates, butin other examples the torque converter turbine may rotate before thedriver tips-in.

At time T11, the driver applies the accelerator pedal (not shown) tolaunch the vehicle. The torque converter impeller speed is slowly rampedto increase the torque transferred from the torque converter impeller tothe torque converter turbine in response to the tip-in. The torqueconverter lockup clutch also begins to close but no torque istransferred since the torque converter lockup torque capacity is low.Thus, torque transfer via the torque converter lockup clutch response isdelayed. The increase in torque converter impeller speed also providescompensation for the delay of torque transfer via the torque converterlockup clutch by temporarily increasing torque transfer through thetorque converter hydraulic torque path.

At time T12, the torque converter lockup clutch torque transfer delayhas ended and the torque converter turbine begins to accelerate bytorque provided through the torque converter friction path (e.g., thetorque converter lockup clutch). The torque converter impeller speedcontinues to ramp up and the ramp rate may be proportional to the driverdemand torque or accelerator pedal position.

At time T13, the torque converter turbine speed reaches the torqueconverter impeller speed. The torque converter impeller and motor and/orengine exit speed control mode and enter torque control mode wheredriver demand torque is the basis for adjusting torque converterimpeller torque. Further, the torque converter lockup clutch torquecapacity is increased to fully lock the torque converter lockup clutch,thereby increasing torque converter efficiency.

In this way, torque converter impeller speed may be ramped in responseto increased driver demand torque so as to increase torque transferredvia a hydraulic torque path at least until the torque converter lockupclutch delay (e.g., a time from commanding the lockup clutch to when thelockup clutch reaches the commanded position) is over. Further, torqueconverter impeller speed control mode may be exited and a torqueconverter impeller torque control mode entered in response to torqueconverter turbine speed being within a threshold speed of torqueconverter impeller speed.

Referring now to FIG. 15, plots of torque converter impeller speed andtorque converter turbine speed during vehicle launch is shown. For thefirst plot at the top of FIG. 15, the horizontal axis represents timeand time increases from the left to right side of the figure. Thevertical axis represents speed and speed increases in the direction ofthe vertical axis arrow. Line 1502 represents a baseline engine idlespeed and line 1504 represents a minimum transmission pump speed toprovide a desired transmission fluid pressure. Dashed line 1506represents torque converter impeller speed. Solid line 1508 representstorque converter turbine speed. For the second plot from the top of FIG.15, the horizontal axis represents time and the vertical axis representtorque converter clutch torque transfer capacity or torque converterslip speed. Dashed line 1512 represents torque converter lockup clutchtorque transfer capacity, and solid line 1510 represents torqueconverter slip speed. The time scales of the first plot and the secondplot represent the same time, and the vertical lines T15-T18 representtime of interest in the launch sequence.

Before time T15 the torque converter impeller speed and torque converterturbine speed are zero indicating that the engine and motor are stopped.Further, the vehicle is stopped and the torque converter lockup clutchis open as is indicated by the lockup clutch torque transfer capacitybeing low. At time T15, the driver releases a vehicle brake pedal (notshown) in preparation for vehicle launch. Shortly thereafter, the torqueconverter impeller speed is accelerated while the torque converterimpeller and motor and/or engine are in a speed control mode in responseto vehicle operating conditions while the torque converter lockup clutchremains open. In this example, the torque converter turbine does notrotate when torque converter impeller accelerates, but in other examplesthe torque converter turbine may rotate before the driver tips-in.

At time T16, the driver applies the accelerator pedal (not shown) tolaunch the vehicle. The torque converter impeller and motor and/orengine are transitioned into a torque control mode where torqueconverter impeller torque is based on driver demand torque. The torqueconverter impeller speed is allowed to accelerate, thereby increasingtorque through the hydraulic torque path of the torque converter. Thetorque converter lockup clutch also begins to close but torque is nottransferred from the torque converter impeller to the torque converterturbine since the torque converter lockup clutch does not closeimmediately. Thus, torque transfer via the torque converter lockupclutch is delayed. The increase in torque converter impeller torqueprovides compensation for delay of torque transfer via the torqueconverter lockup clutch by temporarily increasing torque transferthrough the torque converter hydraulic torque path.

At time T17, the torque converter lockup clutch torque transfer delayhas ended and the torque converter turbine begins to accelerate bytorque provided through the torque converter friction path (e.g., thetorque converter lockup clutch). The torque converter impeller and motorand/or engine transition from torque control mode to speed control mode.Another option is to place motor/engine in torque control mode and letthe converter clutch control the desired impeller speed or slip asdescribed by FIG. 10. The torque converter impeller speed is reduced toreduce the amount of slip and torque transfer though the torqueconverter hydraulic torque path.

At time T18, the torque converter turbine speed reaches the torqueconverter impeller speed. The torque converter impeller and motor and/orengine exit speed control mode and enter torque control mode wheredriver demand torque is the basis for adjusting torque converterimpeller torque. Further, the torque converter lockup clutch torquetransfer capacity is increased to fully lock the torque converter lockupclutch, thereby increasing torque converter efficiency.

In this way, torque converter impeller speed may be transitioned betweentorque and speed control modes to provide increased wheel torque andimproved torque converter efficiency. Further, torque converter impellerspeed control mode may be exited and a torque converter impeller torquecontrol mode entered in response to torque converter turbine speed beingwithin a threshold speed of torque converter impeller speed.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.Further, the methods described herein may be a combination of actionstaken by a controller in the physical world and instructions within thecontroller. At least portions of control methods and routines disclosedherein may be stored as executable instructions in non-transitory memoryand may be carried out by the control system including the controller incombination with the various sensors, actuators, and other enginehardware. The specific routines described herein may represent one ormore of any number of processing strategies such as event-driven,interrupt-driven, multi-tasking, multi-threading, and the like. As such,various actions, operations, and/or functions illustrated may beperformed in the sequence illustrated, in parallel, or in some casesomitted. Likewise, the order of processing is not necessarily requiredto achieve the features and advantages of the example embodimentsdescribed herein, but is provided for ease of illustration anddescription. One or more of the illustrated actions, operations and/orfunctions may be repeatedly performed depending on the particularstrategy being used. Further, the described actions, operations and/orfunctions may graphically represent code to be programmed intonon-transitory memory of the computer readable storage medium in theengine control system, where the described actions are carried out byexecuting the instructions in a system including the various enginehardware components in combination with the electronic controller

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A driveline operating method, comprising:accelerating a torque converter impeller to a desired speed in responseto release of a brake pedal; and maintaining a torque converter impellerspeed at the desired speed in the presence of an increase in driverdemand torque until a torque converter turbine speed is within athreshold speed of the torque converter impeller speed.
 2. The method ofclaim 1, where a motor operating in a speed control mode rotates thetorque converter impeller.
 3. The method of claim 1, where the desiredspeed is a minimum transmission pump speed to provide a desiredtransmission fluid pressure.
 4. The method of claim 1, where an engineoperating in a speed control mode rotates the torque converter impeller.5. The method of claim 1, further comprising at least partially closinga torque converter lockup clutch while maintaining the torque converterimpeller speed at the desired speed.
 6. The method of claim 5, furthercomprising increasing the torque converter impeller speed in response tothe torque converter turbine speed being within the threshold speed ofthe torque converter impeller speed.
 7. The method of claim 6, furthercomprising fully closing the torque converter lockup clutch in responseto the torque converter turbine speed being within the threshold speedof the torque converter impeller speed.
 8. A driveline operating method,comprising: accelerating a torque converter impeller to a desired speedin response to a brake pedal release; maintaining a torque converterimpeller speed at the desired speed until an increase in driver demandtorque; entering a torque converter impeller torque control mode duringa torque converter lockup clutch delay period; and entering a torqueconverter impeller speed control mode in response to the torqueconverter lockup clutch delay period elapsing.
 9. The method of claim 8,further comprising entering the torque converter impeller torque controlmode in response to the torque converter impeller speed being within athreshold speed of a torque converter turbine speed.
 10. The method ofclaim 8, where the desired speed is a base engine idle speed.
 11. Themethod of claim 8, where the desired speed is a minimum transmissionpump speed to provide a desired transmission fluid pressure.
 12. Themethod of claim 8, where the torque converter lockup clutch delay periodis from a time when a torque converter lockup clutch is commanded to atime when the torque converter lockup clutch is at a commanded force orposition.
 13. The method of claim 8, further comprising fully locking atorque converter lockup clutch in response to the torque converterimpeller speed being within a threshold speed of a torque converterturbine speed.
 14. The method of claim 8, further comprising at leastpartially closing a torque converter lockup clutch in response to anincrease in driver demand.
 15. The method of claim 14, furthercomprising adjusting the torque converter lockup clutch in response to avirtual torque converter impeller speed.
 16. A driveline, comprising: anengine; a motor; a transmission including a torque converter and atorque converter lockup clutch, the transmission coupled to the motor;and a controller including executable instructions stored innon-transitory memory for adjusting the torque converter lockup clutchin response to a virtual torque converter impeller speed that isdifferent from an actual torque converter impeller speed.
 17. Thedriveline of claim 16, further comprising additional instructions tooperate the motor in a speed control mode in response to driver demandtorque being less than a threshold torque during a vehicle launch beforea torque converter turbine speed is within a threshold of the actualtorque converter impeller speed.
 18. The driveline of claim 17, furthercomprising additional instructions to operate the motor in a torquecontrol mode in response to driver demand torque being greater than thethreshold torque during the vehicle launch before the torque converterturbine speed is within the threshold of the actual torque converterimpeller speed.
 19. The driveline of claim 16, where the virtual torqueconverter impeller speed is based on a driver demand.
 20. The drivelineof claim 16, further comprising instructions to select a torqueconverter impeller operating mode in response to a driver demand.